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ture meters have delicate components, which are sensitive to impact, extremes of temperature, dampness and dust. Probes have to be be kept clean. Oils and grime can have an insulating effect and distort readings. Left uncovered, the pins could take an inadvertent reading of your personal moisture content. All these factors make a protective case particularly valuable with these tools. Oven- To compare the accuracy of the meters I'd collected, I took readings on three samples of wood-hard maple, black walnut and white oak. I cut the samples from the middle of air-dried boards that had been kept under cover for at least three years. I took multiple readings with each meter and averaged them. dry When I had used all the meters and recorded their readings, I determined the actual moisture content of the samples with the standard oven test: I cut two pieces from each sample (1 in. thick by about 3 in. by 4 in.) and put them in the oven at 2000• With a sensitive electronic scale, I measured the starting weight and kept checking and logging my findings until the samples quit losing weight. (It's helpful to marry someone who doesn't mind you tying up the oven for a few days.) I calculated the moisture content of the original sample by dividing the weight lost by the final dry weight. A piece of wood that weighed 6 oz. wet and 5 oz. oven dry is considered to have had a 20% moisture content when wet: (6-5) .;. 5 = Calculating wood movement by Christian Becksvoort My moisture meter is one of the most important tools I own. Whenever I fit a door, drawer or back, I have to know the moisture content (MC) of the wood. If I know the wood's moisture content and the total anticipated change in moisture content, from the driest January to the most humid August, I can accurately project the gap required above drawers of any given size. I keep two cards (see the chart at right) with my moisture meter, which tell me the total wood movement for quartersawn and flatsawn cherry for an 8% change in moiscontent, ture D S (e.g., someone moving from Phoenix as till = = till = derstanding Wood, p. 74; use tangential shrinkage for flatsawn and radial shrinkage for quartersawn.) = 74 Fine Woodworking well as a worst-case scenario Seattle) of 10% change in moisture content. How did I come up with these magic fig- to ures? I sat down with my calculator and a simple formula and figured each change in dimension. The formula listed below can be found in Understanding Wood by R. Bruce Hoadley (The Taunton Press, 1980): D x S (LlMC .;. FSP) where: Change in dimension Initial dimension Total shrinkage percentage (from Unl-.MC Some examples: How much movement = can be expected for a flatsawn cherry board 10 in. wide if the MC is expected to fluctuate from 14% to 4%? The dimension change The same board exposed to an MC change from 12% to 6%: and till = till = till = l-.D = For each species, S remains constant for %2 board (in.) 1 2 3 4 5 6 7 8 9 10 11 12 Notes: * 10 (.071) (.12-.06 .;. .28) (.71) (.21) in. 10(.071) (,14-.04 .;. .28) (.71) (.36) = = Width of .0406 .0608 .081 1 .1014 .1217 .1420 .1623 .1826 .2028 .2231 .2434 MC .2556 in. will 8% change in Change in width of flatsawn board (.071) .0203 = 11. 3A6 Y,6** % be just over MC· .1491 or between �64 .0106 .021 1 .0317 .0423 .0528 .0634 .0740 .0846 .0951 .1057 .1 163 .1268 moisture content 1f4 in. in. pressed as a decimal; must be less than the fiber saturation point (FSP) of 28% FSP = Fiber saturation point, use 28% .2 evaluation Then I compared the meter readings (after the required species corrections) to the oven-derived moisture content. For each unit, I determined the variance from the oven-dried figures, and that's the figure I used in the chart. ing within 145% of the oven-dry result. It would be a mistake to consider the figures I arrived at to be an absolute measure of the accuracy of the meters. What the process told me was that most of the meters are in the same ballpark; therefore, the features and accessories each one has to offer are the most important factors in choosing between them. I must say, though, I was pleased to discover that the Lignomat Mini-Ligno I have been using for the last 10 years has been accurate within 15%. I am not sure I have told the truth that much myself. I found I liked something about nearly all the meters I surveyed. As But if I were to go out and buy a meter right now, I'm pretty sure I'd go for the Wagner L 606. It has EMF technology at a reasonable price and wonderfully extensive correction tables. It's easy to use, and it's lightweight and compact. A real winner. If I were choosing at the low end of the price spectrum, I'd probably get the Electrophysics MT 80. You get good species corrections, good accuracy, and with their alligator clip setup, you can get deep readings when you need them. 0 John Sillick is a teacher and woodworker in Lyndonville, NY a group, the meters did well, stayChange in moisture content ex- flatsawn and constant for quartersawn. S is the percent of shrinkage from the green state (considered to be 28% MC) to oven dry (0% MC). These values (for domestic as well as imported species) can be found in Understanding Wood or in The Wood Handbook: Wood able from the Superintendent of Documents, United States Government Printing Office (Washington, D.C. 20402) or your local library. Prod width of quartersawn board (.037) 0 Christian Becksvoort is a contributing editor to Gloucester, Maine. FWW Movement of cherry (from 6% to 14 %) 10% change in Change in Change in width of flatsawn board (.071) .0253 Y,6 31.12 % ** .0507 .0761 .1014 .1268 .1521 .1775 .2028 .2282 .2536 .2789 .3043 0/64 51.12 MC Approximate fractional equivalent 15/64 0/.6 and a furnituremaker in rial; Agricultural, Handbook #72, Forest ucts Lab, 1976. This handbook is avail- as an Engineering Mate- (from 4% to 14%) Change in width of quartersawn board (.037) .0132 .0264 .0396 .0528 .0661 .0793 .0925 .1057 .1189 .1321 .1454 .1586 0/64 7/64 51.12